<?php
function nslookup($domain, $nameserver) {
    $cmd = "nslookup -querytype=A $domain $nameserver";
    $output = shell_exec($cmd);
    return $output;
}

if(isset($_POST['domain']) && isset($_POST['nameserver'])){
    $domain = $_POST['domain'];
    $nameserver = $_POST['nameserver'];
    $result = nslookup($domain, $nameserver);
    echo nl2br($result);
}
?>

<form method="post" action="">
    <textarea name="domain" placeholder="Enter domain here" rows="5" cols="40"></textarea><br>
    <textarea name="nameserver" placeholder="Enter nameserver here" rows="5" cols="40"></textarea><br>
    <input type="submit" value="Submit">
</form>